1. Field of the Invention
The present invention relates to improvements on previous systems for ultrasonically cleaning irradiated nuclear fuel assemblies.
2. Description of Related Art
Ultrasonic energy cleaning has been successfully employed to remove crud deposits from irradiated nuclear fuel assemblies. The initial commercial motivation for removing these deposits from pressurized water reactor (PWR) fuel has been to mitigate crud induced power shift (CIPS), also sometimes referred to as axial offset (AO) or axial offset anomaly (AOA). CIPS is a phenomenon in which deposits form on the fuel rod cladding due to the combination of local thermal and hydraulic conditions and primary side fluid impurities characteristic of the reactor and primary system. Certain compounds residing in these deposits act as a poison to the nuclear reaction and, because the deposits are typically thicker in the top portion of the core, cause an abnormal power distribution along the axis of the core, reducing available margin for certain types of operating conditions. As a result of CIPS, some power plants have been forced to reduce their reactor power level and hence electrical output for extended periods, which has been expensive for the operating utilities.
In addition to mitigating CIPS, removal of crud deposits from irradiated fuel assemblies produces some other favorable results in nuclear plants of any design, including but not limited to (1) reducing the total crud inventory (“source term reduction”) of plant primary systems, which leads to lower radiation dose rates for plant personnel (ALARA), (2) improving fuel inspectability, and (3) reducing potential for spread of radioactive contamination during fuel transport, storage, and reprocessing.
Electrical generation at power plants is routinely interrupted to perform certain tasks that cannot be performed online, such as certain maintenance tasks, inspections, and reactor refueling. During the refueling of a main reactor, the fuel assemblies are removed from the reactor core and stored underwater near the reactor or in the plant's spent fuel pool. Later in the outage, some of the irradiated fuel assemblies are put back into the reactor core, while others are left in the spent fuel pool so that fresh assemblies can be loaded in their place. Existing ultrasonic fuel cleaning systems operate in the window when the fuel assemblies are in the spent fuel pool and available for cleaning. Because fuel offload is generally performed during critical path outage time, it has been more economical to offload fuel first, then go back and clean it after the offload is complete.
Efforts by owners and operators of power plants to reduce outage lengths are resulting in a reduced amount of time available for cleaning fuel assemblies for those plants which perform a full core offload as part of refueling. Furthermore, it has become standard practice in the boiling water reactor (BWR) community to keep a significant percentage of the fuel in the reactor vessel throughout the refueling outage, in practice known as “fuel shuffling.” It would be beneficial if a fuel cleaning system were capable of cleaning fuel assemblies during the fuel movements without a significant impact on the time required to move (shuffle or offload) fuel. Such is the case with the invention presented herein.